This invention relates generally to systems for operating intermediate-link frequency converters, and more particularly, to a system wherein energy is conducted in two directions via two antiparallel-connected controlled converters which are coupled to an intermediate-link converter by an inverter in a three-phase bridge circuit.
A known intermediate-link converter which utilizes an intermediate DC link is described in the text Thyristoren, by Heumann and Stumpe, 3rd Edition, 1974, page 198. It is often desirable to operate a frequency converter in both energy directions such that a load is supplied electrical energy from a supply voltage source, and the load may function as a generator to feedback electrical energy to the source. In this manner, electrical motors can be braked almost without loss by feeding back the energy to the supply voltage source. Such energy-recovering circuits have been provided heretofore only for converters having a DC intermediate link. In such known circuits, the direction of the current in the intermediate link is preserved, and only the voltage in the intermediate link is reversed. This operation is effected by simply shifting the control angle of the inverter in the manner described in the known reference Thyristor Handbuchs, by Hoffmann and Stocker, 4th Edition, 1976, page 348. In this known system, the inverter operates as a rectifier and a continuous transition occurs between the feeding-in and feedback modes of operation. Thus, in intermediate-link converters of the type wherein a DC intermediate link is utilized with an impressed voltage thereon, only the intermediate-link current, but not the intermediate-link voltage, can be reversed. A separate rectifier is required to conduct the reverse current so that the intermediate-link converter comprises two antiparallel-connected rectifiers on its input side. When the system enters the recovery, or feeding-in, mode of operation, this second rectifier must be switched on.
It is, therefore, an object of this invention to provide a system for operating an intermediate-link converter wherein the antiparallel-connected rectifiers on the input side are switched over as soon as the conditions for recovery are present.
It is a further object of this invention to provide a circuit arrangement which implements the above-mentioned method.